The invention concerns wear resistant mechanical devices in general and, in particular, surface hardened sprags and rollers for overrunning clutches.
The most common form of wear in mechanical devices is adhesive and mild abrasive wear. There is a complex relationship between near surface structures and interaction between two faces in sliding contact. A magnified cross-section of a metal surface reveals myriad peaks (asperities) and valleys. Asperities will adhere to one another when faces are in sliding contact. Microscopic pieces of the less durable surface are plucked out (adhesive wear) which subsequently leads to more damaging abrasive wear. The dislodged particles work harden and combine with other hard particles to gouge the contacting surfaces.
Furthermore, there are small grains that form a lattice of different defects in the subsurface structure of the metal device. One defect that is especially troublesome is called a dislocation. As dislocations move under applied loads, surface microcracks are initiated that eventually propagate and aggravate particle loss.
Both coatings and heat treatments can easily improve wear resistance. Titanium-nitride coatings and hard chromium coatings, for example, resist wear better than most metal substrates. However, the applied thickness of material changes dimensions and can even delaminate in severe service. Heat treatments provide material hardness at an atomic level, so that delaminating is not a problem. The process temperatures, though, can distort precision parts.
Ion implanting, which takes place at room temperature, is a viable alternative where dimensionality and distortion are concerns. In one process, electrons emitted from a hot tungsten filament collide with nitrogen atoms and strip an electron to form an ion. The nitrogen ions are electrostatically extracted from the ion source and focused into a beam. The beam is accelerated to approximately 100 KeV and focused at the surface to be treated. The ions strike the surface and become embedded as a part of the surface. The embedded nitrogen ions act as atomic anchors by strongly coupling the structural defects such as dislocations. Injecting ions into a near-surface region induces compressive stresses (similar to the benefit of shot peening) that reduce the tendency of surface cracks to open. With tool steels, some nitrogen ions even bond with alloying elements (e.g., chromium or vanadium), forming extremely hard nitrides. These also protect the surface from fracturing. Further, certain oxide films eliminate severe adhesion and reduce the coefficient of friction. By modifying the composition of the uppermost layers of the object material, ion implanting reduces the chemical affinity of surfaces in contact, promotes normal oxide growth, and strengthens the metal oxide/metal interface, allowing a naturally formed surface oxide to serve as solid lubricant.
Although nitrogen ions penetrate less than one micron deep, during wear the implants debond and retreat from the wear front to form quasi-nitrides. This provides a persistence of wear resistance that is ten to fifty times deeper than the initially treated zone.
Ion implantation is considered a low temperature process. Because the atoms are implanted mechanically, there is no need for the application of high temperature to produce a thermal defusion of the atoms into the metal. The only heating that occurs is that due to the energetic atoms colliding with the atoms of the base material. The maximum temperature in the workpiece, which seldom exceeds four hundred degrees F., can be minimized by controlling the rate of implantation.
One mechanical device which typically requires a hardened surface for a use is a high carbon steel or steel alloy sprag or roller to be used as a wedging strut between concentric races of an overrunning clutch. Such devices were typically hardened by a chromizing or chromalizing treatment. It was found that the hardened surface could be improved by subjecting the device to a nitriding or carbonitriding heat treat operation prior to the chromizing treatment. In this process, the devices are thoroughly cleaned to eliminate all surface contamination and are heated in a furnace at 200.degree. F. to 250.degree. F. to drive off all moisture present. The devices are placed in baskets in a heat treating furnace and are heat treated in the presence of raw ammonia at a temperature in the general range of 1535.degree. F-1500.degree. F., for a soaking time at temperature depending upon the desired body hardness characteristic of the steel. Quenching and further cooling follow under controlled conditions, and drawing to relieve stress.